1. Field of the Invention
The present invention is directed to an non-volatile random access memory (NVRAM) cell and a method for fabricating thereof, and more particularly, to an NVRAM cell that has a at least one sharp tip or spike extending into a source or drain of the NVRAM cell.
2. Discussion of the Prior Art
Smart cards and mobile applications require low power NVRAM cells, such as electrically erasable programmable read only memories (EEPROM), where their storage state, e.g., logic high or low, is retained after power is removed. Advanced complementary metal oxide semiconductor (CMOS) cells and processes have been aggressively scaled down to reduce cell size and power supply voltage requirements for reading and writing and erasing the NVRAM cells. Scaled NVRAM cells require high voltages for programming or erasing the NVRAM cells. The high voltages associated with programming and erasing the NVRAM cells incompatible with CMOS device scaling.
FIG. 1 shows a schematic of a typical NVRAM cell 100, having a floating gate 110, a control gate 120 and two drain/source terminals 105 and 115. The terminal 105 is the bitline of the cell 100, it may be connected to decode circuitry. For simplicity, hereinafter, the bitline B/L terminal 105 will be referred to as a drain, while the other terminal 115 will be referred to as a source.
Depending on voltages applied to the bitline B/L terminal 105, the control gate 120 and the source 115, the floating gate 110 is charged (written) or discharged (erased). The discharge occurs only through the source terminal 115. A floating gate 110 of the NVRAM cell 100 stores charges.
The floating gate 110 is covered by a dielectric insulating layer 130, such as an oxy-nitride-oxide (ONO) layer, which is a combination of oxide, nitride, and reoxidation of the nitride, in order to retain the stored charges after the completion of writing or erasing operations. The dielectric insulating layer 135 located between the floating gate and the underlying substrate 140 is referred to as a tunnel oxide, which is normally thin to allow electron tunneling between the floating gate and the source (or the drain) of the floating gate transistor 100.
The present disclosure relates to a stacked gate nonvolatile memory cell, where the signal is stored on the floating gate. Those skilled in the art will recognize that these cells can be operated in a number of ways. Two of the best methods used for cell operation are described below. These methods are not intended to be an exhaustive list, however, the practitioners of the art will recognize that they define two major classes for which other operation modes will derive.
Channel Hot Electron Write
To write the cell 100 in the Channel Hot Electron mode of operation, voltages are applied to the source 115, the drain 105, and the control gate 120 to turn on the channel region 125. In this mode the current through the channel 125 generates hot electrons near the drain region 105. Some small fraction of these hot electrons have enough energy to be injected into the floating gate 110 across the dielectric film 135 separating the floating gate 110 from the channel region 125 and the source 115 and the drain 105 regions. This dielectric film is typically silicon dioxide and is referred to as the tunnel oxide.
A typical set of voltages for an NFET memory cell is putting the source 115 to ground potential 0 volts, placing the drain 105 at a positive voltage typically e.g., 5 volts and placing a high potential on the control gate 120 e.g., 10 volts. The voltage on the floating gate 110 defines the conduction of the conduction region. The floating gate 110 is analogous to the gate on a standard CMOS transistor. However, since it is electrically isolated, the voltage applied to the control gate 120 is coupled to the floating gate 110 via a ratio of capacitance between the inter-gate dielectric 130 and the tunnel oxide 135, and the bias conditions of the adjacent regions.
Tunnel Write
Writing the cell 100 using Tunnel Write mode of operation involves biasing the drain region 105 and the control gate 120 in such a manner as to create an electric field in the region of the tunnel oxide between the drain 105 and the floating gate 110, such that electrons will tunnel from the drain region to the floating gate. In the example of the NFET memory cell 100, the drain region 105 could be set at ground 0 volts, the source region 115 could be set at 5 volts, while the control gate 120 is biased to a high voltage e.g., 15 volts. As was the case with Channel Hot Electron mode of operation, the actual voltage on the floating gate 110 is defined by capacitive coupling between the floating gate 110 and all adjacent regions. Note that the currents required to Tunnel are much less than those required to inject electrons by Channel Hot Electrons.
Tunnel Erase
The erase operation is identical in the Channel Hot Electron Write mode and in the Tunnel Write mode of operation. To erase the cell 100, the source region 115 is biased to a relatively high potential, and the control gate 120 is biased to a negative potential such that high electric field is produced across the tunnel oxide which will allow tunneling of electrons from the floating gate 110 to the source 115. For example, in a NFET memory cell 100 the source 115 may be biased to 8 volts while the control gate 120 is biased to negative 6 volts.
As was the case during the Tunnel Write, the voltage on the floating gate 110 defines the field across the tunnel oxide. Since floating gate 110 is electrically isolated, the voltage is defined by capacitance ratios between the inter-gate dielectric and the tunnel oxide considering the voltages applied to the adjacent regions, i.e., the source 115, the drain 105, and the channel 125. Writing the cell is an act of adding electrons to the floating gate 110, while erasing the cell is an act of removing electrons from the floating gate 110. It may also be shown that channel region 125 is conductive in the case of an erased NFET cell, while it does not conduct current readily in the written state.
One method to reduce the high NVRAM programming voltages is scaling or reducing the thickness of the tunnel oxide 135 located between a floating gate 110 and the source 115. However, thin tunnel oxides have been associated with the loss of the stored charge on the floating gate 110, so called retention fails. Thus, such cell cannot be used as non-volatile memory. Accordingly, there is a need to provide an NVRAM cell that operates properly at low voltages, without further reducing the thickness of the tunnel oxide and thus, avoiding problems associated with thinner tunnel oxide layers.